The evidence is more than good enough, so red tags and/or red flags can be left on the profiles by you or any member.
Correction: "red flags" (I assume you mean type 2 or 3) can only be created by actual victims of the scammer.
See the Flag creation page:
[type 1] Due to various concrete red flags, I believe that anyone dealing with this user has a high risk of losing money. (This flag will only be shown to guests/newbies.)
[type 2] This user violated a casual or implied agreement with me, resulting in damages.
[type 3] This user violated a written contract with me, resulting in damages.